The possibility of focusing an electromagnetic wave on zones of very small area has been used, for a long time, for applications in a very wide variety of fields. The following may be mentioned: microscopy; production of optical detectors or sensors; production of optical systems for writing and/or reading data on a recording medium; and, more generally, all applications in which light is used for locally modifying or probing a focusing zone or the material that is located thereat. In addition, the course in miniaturizing devices and systems, together with the advent of nanosciences and nanotechnologies, requires an increase in the ability of optical probes being focused onto ever smaller areas.
However, focusing an electromagnetic wave by a conventional far-field optical system is normally limited by the Rayleigh criterion to a radius r equal to λ/2n sin θ, in which r is the size of the focal point, λ is the wavelength of the electromagnetic wave, n is the optical index of the material in which said wave propagates, and θ is the maximum opening angle of the focusing lens system. To focus a wave onto areas of small as possible radius, several approaches are generally followed.
The first consists in increasing the maximum numerical aperture NA, equal to n sin θ. This is accomplished either by immersion in a liquid of high optical index or by immersion in a solid material, again of high optical index, having a hemispherical or superhemispherical lens. Such a lens is referred to as an SIL (solid immersion lens), the focal point thereof lying in the plane of the hemisphere or the superhemisphere. In practice, these techniques enable the light to be focused on a focal spot smaller by a factor n or n2 than a conventional system while maintaining a transmission close to 100%, the factor depending on the shape of the lens. The limitation of this technique is tied to the optical index of the material, which does not exceed a few units.
The second possible approach consists in concentrating this field by what are called near-field optical methods. These methods exploit the natural localization of the electromagnetic field in the immediate vicinity of a nano-object, in the form of a nonpropagating field due to diffraction. The term “nano-object” is understood to mean an object having at least one of its dimensions smaller than a few tens of nanometers. The geometry, the spatial distribution and the amplitude of this field are determined, on the one hand, by the nature, the geometry and the size of the nano-object and, on the other hand, by the polarization and wavelength characteristics of the diffracted light. The operation is as follows: an incident wave is sent onto a nano-object that diffracts this wave, its size being small compared with the wavelength. The resultant field has a conventional propagating component and a nonpropagating component that remains localized close to the nano-object and called the near field. This near field can then be modified by a second object, again of small size compared with the wavelength. The modification is either a diffraction, a scattering or a modulation of the field. Many applications use the generation and detection of this localized field for writing memory cells, for characterizing, exciting and detecting objects, generally of nanoscale dimensions spatially localized in this near field created by the first nano-object, near-field microscopy, etc. In practice, two types of nano-object are used to generate the localized field.
The first type of nano-object is a nanoscale hole in an opaque screen, generally a metal screen. It can be produced in planar geometry or in a metal coating on a dielectric support, such as an optical fiber or a waveguide. In this geometry, the size of the focal spot depends only on the size of the hole. The near field generated in transmission is used by these holes so as to optimally circumvent the incident wave, offering a good signal-to-noise ratio. In the case of metal screens, the enhancement effect, by plasmon mode coupling, may also be advantageously used to further increase the signal-to-noise ratio between the local field and the propagating field. The main limitations of this approach stem, on the one hand, from the low transmission obtained, which is proportional to the factor (a/λ)4 where a is the diameter of the hole, and, on the other hand, from the penetration depth of the light wave into the opaque material constituting the screen, said depth being tied to the skin depth in metals. Theoretically, the resolution is limited to about 15 to 20 nm. This type of structure has been very widely used, for example for applications of the near-field optical microscopy type with an aperture. The techniques used to produce these fiber-optic-based tips are generally not compatible with standard microelectronic processes and these tips are therefore not very reproducible. To remedy this situation, certain standard processes have been proposed. The reader may refer in particular to the article by P. N. Minh et al. published in Review of Scientific Instruments—Vol. 71, 3111 (2000). However, the size of the nanoscale aperture in the screen cannot be controlled to better than 50 nm.
The second approach consists in using a single nano-object of defined geometry, such as a nanosphere, a nanodisk or a paraboloid having at least one confined dimension so as to concentrate the near field thereof. In this approach, the skin effect is not a limitation and the field may be potentially confined over very small dimensions. Likewise, the transmission is generally no longer a problem when this approach is considered. However, it is necessary to extract the confined-field signal from the incident signal. This is accomplished by modulation techniques, which involve either the physical manipulation of these nano-objects, something which is often difficult, or the use of the enhancement effect via surface plasmon modes in the case of metal structures. This geometry is very widely used to produce sensors and detectors and for near-field optical microscopy without an aperture. However, the manipulation of single zero-dimensional nano-objects remains difficult and in practice nanoscale objects having at least one macroscopic dimension are more often used.
To combine the advantages of the two approaches, that with an aperture and that without an aperture, so as to maintain the ultimate resolution with a favorable signal-to-noise ratio, it is known to add a nano-object to a nano-aperture. In this regard, the article by T. H. Taminiau et al., published in Nano Letters Vol. 7, 28 (2007) entitled “λ/4 resonance of an optical monopole antenna probed by single molecule fluorescence” may be mentioned. In this case, a metal antenna is attached to an SNOM (scanning near-field optical microscope) tip or NSOM (near-field scanning optical microscope) tip having a conventional aperture using focused ion beam techniques. The limitations of this structure are numerous. Firstly, the transmission from the conventional SNOM optical tip remains low, as in the case of the abovementioned tips with an aperture. Secondly, the nano-object serving as antenna is produced by etching the metal mask with a focused ion beam. These production techniques cannot be easily exploited for production in parallel of these focusing heads using mass fabrication techniques.
It is also known to use SIL lenses for near-field excitation or collection of the aperture type, while still maintaining a transmission close to 100%. The generation of evanescent waves at the focal point of the SIL lens it is then tied to the total internal reflection at the plane interface of the lens due to its geometry. This has been advantageously employed in many applications:                of the microscopy type: the reader may refer to the publication by S. M. Mansfield et al., published in Applied Physics Letters Vol. 57, 2615 (1990);        of the optical recording type: see the same authors, Optics Letters 18, 305 (1993);        or else photolithography, see article by L. P. Ghislain et al., Applied Physics Letters 74, 501 (1999).        
Such SIL lenses have also been combined with a pyramidal or conical tip. These solutions have been described in U.S. Pat. No. 6,441,359. This tip is produced on the focal point side of the lens, enabling this lens to be scanned, close to the measured specimen, over a distance close to the wavelength. Such tips have a typical radius of curvature of 500 nm and are produced in the same material as that constituting the SIL. It is also known to add a metal coating to this tip, said coating being pierced by a nanoscale hole serving to limit the size of the focal point. The main drawback of these tips is their low aspect ratio—the apex cone angle is typically 65° so as to maintain the focusing effect. This angle is very unfavorable for obtaining a high topographical resolution in applications of the near-field microscopy type. These tips may have a metal coating but they then suffer from the same limitations as SNOM tips with conventional apertures as described above.